Automotive coatings containing metallic pigments such as aluminum flake are generally used to obtain the glossy lustrous appearance which is currently in-demand in the automotive market. There has been a effort in the automotive industry to reduce atmospheric pollution due to volatile solvents emitted during the painting process without sacrificing the appearance of the coating system. One approach to this end has been to develop waterborne coating compositions. Waterborne coating compositions, however, are not without disadvantages. For example, aluminum flakes which are used in metallic paints react with water and release hydrogen gas, resulting in unstable compositions.
Prior art efforts to minimize this aluminum-water reaction include treatment processes such as solvent and chromium treatments that render the aluminum surface substantially inert. Chromium treated aluminum pigments are available from Obron Atlantic Corporation under the trademark Stapa Hydrolux. Coatings made with chromium treated aluminum flake pigment are stable with respect to gassing, but the treated aluminum pigment is very expensive.
U. S. Pat. Nos. 5,034,556 and 5,091,451 disclose reaction products of alpha-aminomethylene phosphonic acids and epoxy compounds and their use in metallic pigment-containing aqueous coating compositions as gassing inhibitors. However, these coating compositions, although essentially non-gassing, exhibit marginal pump stability; that is, the high shear stresses to which the composition is subjected during pumping through equipment cause the composition to break down somewhat, losing some of its rheological properties. The resulting coating has poor appearance properties.
U. S. Pat. No. 4,621,112 discloses reaction products of orthophosphoric acid and monoesters thereof with epoxy compounds and their use in metallic pigment-containing coating compositions as gassing inhibitors. However, such compositions exhibit only marginal gassing resistance.
British Patent 1380675 discloses N-acyl aminomethylene phosphonates and their use as flame proofing agents. The British reference does not teach nor suggest the use of such compounds in waterborne coating compositions as gassing inhibitors.
It would be desirable to provide a waterborne coating composition containing metallic pigments which is stabilized against gassing by including an inexpensive organic additive which does not make the coating composition shear sensitive, so as to result in good pump stability.